This invention relates generally to a switching regulator for a power supply and more particularly to a method and apparatus for digitally controlling the output of a power supply by varying the on-off duty cycle of power transistor switches wherein the switches are always turned on into a non-conducting inductor and a fully recovered catch diode.
Engineers involved in the design of computers, digital logic systems, precision controls, and similar applications spend much time and money attempting to design highly reliable power supplies. In addition to the high reliability criteria, considerations such as cost, efficiency, bulk and weight come into play, and specific considerations such as stability, noise and ripple must not be forgotten.
Some of the critical parameters effecting the reliability of the power supply, and ultimately most of the other design goals cited above, are thermal considerations. In general, the highest level of reliability is achieved by operating the electronic components as close to ambient temperature as possible. This governing principle dictates the size of the heat exchanger, and the trade-off involved between cooling system components and efficiency dictates the ultimate weight, bulk, cost and even the approach to circuit design used by the engineers. These thermal considerations are deeply affected by the method of regulation employed in the system.
When a load requires precision control of both voltage and current, and when an outside AC or DC source of energy is used, a system must usually incorporate a power supply using a regulator. Basically, a power supply regulator is either dissipative or non-dissipative. The dissipative regulator absorbs the difference between the input voltage at the source and the unregulated voltage at the load, and the power dissipation is given as (V.sub.in -V.sub.out)*I.sub.L. A non-dissipative regulator stores the excess power in an LC filter and delivers the power to the load in measured intervals. Non-dissipative regulators generally use switching devices to control the output power. During the time the switch is on, power is stored in an energy storage network (the LC filter) and is delivered to the load as required. Ideally, the switching approach exhibits no power dissipation. Since conventional dissipative series or shunt regulators operate the power-regulating transistor in a continuous-conduction mode, large amounts of power are dissipated at high current loads, especially when the input-output voltage difference is large.
The design of modern regulators is aimed at increasing efficiency and operating reliability while saving energy, hence most modern regulators are of the non-dissipative type. Power supply circuits using switching regulators have high efficiency under all input and output conditions. Since the power transistor switch is always either cut off or saturated, except for a very brief transition period between those two states, the switching regulator can achieve good regulation despite large changes in input voltage and maintain high efficiency over a wide range of load currents. The switching regulator regulates by varying the on-off duty cycle of a power transistor switch, and the switching frequency can be made very much higher than the line frequency, the filtering elements used in the power supply can be made small, light weight, low in cost and very efficient. It also becomes possible to drive the switching regulator with very poor filtered DC, thereby eliminating large and expensive line frequency filtering elements. Furthermore, it is possible to design switching regulators with excellent load-transient properties so that step increases in load current cause relatively small instantaneous changes in the output voltage, recovery from which is essentially completed in a few hundred microseconds. Since the overall efficiency of a high frequency switching regulator means less heat dissipated and less bulk, then the size of the heat exchanger may be reduced if the size of the inductors and capacitors is reduced because smaller valued devices can be used. It is not, therefore, unreasonable to see why the switching regulator has become increasingly popular in electrical design, not only in aerospace and defense applications, but in computers, logic systems, control systems, instrumentation and communication areas.
The switching regulator, however, is not without its disadvantages which preclude its use in some applications. The primary power source delivers current to the switching regulator in pulses which, for efficiency reasons, have short rise and fall times. In those applications where a significant series impedance appears between the supply and the regulator, the rapid changes in current can generate considerable noise. This problem can be minimized by reducing the series impedance, increasing the switching time, and/or filtering the input to the regulator. Another problem inherent in the use of switching regulators arises from the circuits response time to rapid changes in the load current. The switching regulator will reach a new equilibrium only when the average inductor current reaches its new steady state value. In order to make this time short, it is advantageous to use low inductor values or else to use a large difference between the input and output voltages.
Since many applications will not tolerate induced transients, the prior art is replete with various methods and apparatus aimed at preventing transient-induced damage, current surges and the like, and many systems have been designed to insure over-voltage protection, under-voltage protection and current limiting. Elaborate filters are often used on the input and output leads to eliminate or minimize electromagnetic or radio-frequency interference. Turn-on transient spikes and turn-off transient spikes often dictate the size of the inductor or filter capacitors, and various trade-offs must be made which limit the regulator's response bandwidth, efficiency and reliability. Several techniques of the prior art recommend completely surrounding the regulator in a shielded enclosure so as to eliminate or minimize noise leakage when the switching regulators are operating in frequencies having detectable harmonics.